The invention relates to wireless local area networks, and in particular, to a protection mechanism therefore providing high performance.
IEEE 802.11 is a wireless local area network standard comprising various purpose protocols. For example, request to send (RTS)/clear to send (CTS) is a medium reservation protocol, and CTS-to-Self is a CTS without a preceding RTS used by 802.11g stations (STAs) to reserve the medium in a basic service set (BSS) requiring “protection”. ERP-OFDM (802.11g) and HS-DSSS (802.11b) are different modulation schemes utilizing the same frequency of 2.4 GHz, therefore “Protection” is required when the BSS is functioning in mixed mode supporting both standards.
FIG. 1 is a timing chart of conventional RTS/CTS protocol. When RTS/CTS is enabled on a particular station, it will refrain from sending a data frame until the station completes an RTS/CTS handshake with another station, such as an access point (AP). A source SRC initiates the process by sending an RTS frame. A destination DST receives the RTS frame and responds with a CTS frame within a short inter-frame spacing (SIFS) interval. SIFS is a predefined pause subsequently 16 μs in 802.11a (this value is different in 802.11 a/b/g/j). When the SRC receives the CTS frame, the DATA frame is confirmed as delivered. The CTS frame also contains a network allocation vector (NAV) that alerts other stations to refrain from accessing the medium while the SRC transmits the DATA frame. The source SRC and the destination DST, can be an AP/STA pair or STA/AP pair, and the RTS/CTS protocol is applicable for both downlink and uplink transmission. The RTS/CTS handshake provides positive control over the use of the shared medium. The primary reason for implementing RTS/CTS is to minimize collisions among hidden stations.
FIG. 2a shows a wireless network environment comprising both 802.11b and 802.11g stations 204 and 206, and one AP 202 supporting both standards. The 802.11b standard is an older version supporting only Complementary Code Keying (CCK) modulation. In addition to compatibility with the 802.11b standard, the 802.11g standard also utilizes Orthogonal Frequency Division Multiplexing (OFDM) modulation. Therefore various schemes are proposed to work with the mixed network environment.
FIG. 2b is a timing chart of a conventional CTS-to-self protocol for the wireless network environment in FIG. 2a. A source SRC initiates the transmission by sending a CTS frame, and then delivers the DATA1 frame within one SIFS interval. After delivering the DATA1 frame, an ACK1 frame is expected from the destination DST within one SIFS interval. If the ACK1 frame is not detected in time, the transmission of the DATA1 frame is deemed a failure. If the ACK1 frame is received as expected by the SRC, another data transmission is initialized, a DATA2 frame is delivered after sending a second CTS frame, and a second ACK2 is expected. The steps recursively loop as long as the DATA frames are available to send, thus the protocol is also referred to as a burst mode. In FIG. 2b, the CTS frame contains an NAV for reserving the medium for a period of time. From the falling edge of the CTS frame to the falling edge of the second ACK2 frame, the NAV protects a total of two DATA frames, two ACK frames, one CTS frame and a plurality of SIFS therebetween. The CTS frame is CCK modulated so all the 802.11b and 802.11g STAs are able to interpret the NAV to keep the medium clear during receipt. Thus, the CCK modulated CTS frames provide mixed mode protection by the NAV therein. In the CTS-to-self mechanism, the source SRC is typically an AP, and the destination DST is a STA. When the BSS does not exceed a predetermined scale, the role of SRC/AP may also be reversed.